To overcome some types of hearing loss, numerous cochlear implant systems—or cochlear prostheses—have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers by way of one or more channels formed by an array of electrodes implanted in the cochlea. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
When a cochlear implant system is initially implanted in a patient, and during follow-up tests and checkups thereafter, it is usually necessary to fit the cochlear implant system to the patient. Such “fitting” includes adjustment of the base amplitude or intensity of the various stimuli generated by the cochlear implant system from the factory settings (or default values) to values that are most effective and comfortable for the patient. For example, the intensity or amplitude and/or duration of the individual stimulation pulses provided by the cochlear implant system may be mapped to an appropriate dynamic audio range so that the appropriate “loudness” of sensed audio signals is perceived. That is, loud sounds should be sensed by the patient at a level that is perceived as loud, but not painfully loud. Soft sounds should similarly be sensed by the patient at a level that is soft, but not so soft that the sounds are not perceived at all.
Hence, fitting and adjusting the intensity of the stimuli and other parameters of a cochlear implant system to meet a particular patient's needs requires the determination of one or more most comfortable current levels (“M levels”). An M level refers to a stimulation current level applied by a cochlear implant system at which the patient is most comfortable. M levels typically vary from patient to patient and from channel to channel in a multichannel cochlear implant.
M levels are typically determined based on subjective feedback provided by cochlear implant patients. For example, a clinician may present various stimuli to a patient and then analyze subjective feedback provided by the patient as to how the stimuli were perceived. Such subjective feedback typically takes the form of either verbal (adult) or non-verbal (child) feedback. Unfortunately, relying on subjective feedback in this manner is difficult, particularly for those patients who may have never heard sound before and/or who have never heard electrically-generated “sound.” For young children, the problem is exacerbated by a short attention span, as well as difficulty in understanding instructions and concepts, such as high and low pitch, softer and louder, same and different. Moreover, many patients, such as infants and those with multiple disabilities, are completely unable to provide subjective feedback.
Hence, it is often desirable to employ an objective method of determining M levels for a cochlear implant patient. One such objective method involves increasing a current level of electrical stimulation applied by a cochlear implant system to a patient until a stapedius reflex (i.e., an involuntary muscle contraction that occurs in the middle ear in response to acoustic and/or electrical stimulation) is elicited. The current level required to elicit a stapedius reflex within a patient (referred to herein as a “stapedius reflex threshold”) may then be used by a clinician as a starting point for determining an M level corresponding to the patient.
Unfortunately, it is sometimes impossible to detect an occurrence of a stapedius reflex in some patients. For example, some patients may not have a functioning stapedius tendon and are therefore incapable of producing a stapedius reflex. Other patients have abnormal middle ear topologies, thereby making it impossible to detect a stapedius reflex using conventional detection techniques.
The possibility of not being able to detect an occurrence of a stapedius reflex in a patient raises various safety concerns, especially for pediatric patients incapable of providing subjective feedback. For example, if the current level of the electrical stimulation is increased very much above the M level for a particular patient, the electrical stimulation may cause pain, discomfort, and/or damage to the patient.